1. Field of the Invention
This invention relates to an auto drive system for a vehicle or a cruise control system for a vehicle.
2. Description of the Prior Art
Japanese published unexamined patent application 61-1545 discloses an auto drive (cruise control) system for a vehicle. The auto drive system of Japanese application 61-1545 includes a potentiometer for detecting the degree of depression of an accelerator pedal, and first and second control devices. The first control device adjusts the degree of opening of a throttle valve on the basis of an output signal from the potentiometer, enabling the vehicle to be driven in response to the depression of the accelerator pedal. When auto drive is required, the second control device adjusts the degree of opening of the throttle valve so as to regulate the speed of the vehicle at a desired speed.
The auto drive system of Japanese application 61-1545 has a problem as follows. After setting of the desired auto drive speed is completed and then the accelerator pedal is released, the accelerator pedal is returned to its undepressed position by a return spring while the throttle valve is held at a desired open position. This positioning of the throttle valve independent from the accelerator pedal is enabled by the absence of a mechanical connection between the accelerator pedal and the throttle valve. Thus, when the vehicle driver requires an interruption of auto drive and a temporary increase in the vehicle speed from the desired auto drive speed, it is necessary for the vehicle driver to depress the accelerator pedal from its undepressed position. During the depression of the accelerator pedal, the vehicle speed is held at the desired auto drive speed by the auto drive system until the accelerator pedal reaches a position corresponding to the desired auto drive speed. Accordingly, during a former stage of the depression of the accelerator pedal, the vehicle speed is insensitive to the movement of the accelerator pedal. This insensitiveness tends to cause the vehicle driver to excessively depress the accelerator pedal.